DRIVING THROUGH FRANCE - LOW EMISSION ZONES.https://www.bbc.

Buy them from French Government website.

https://www.certificat-air.gouv.fr/

The intention is to create more and more low emission areas in the next few years

Sticker covers all areas in France

Sticker lasts lifetime of the car

If you are trying to buy at the last minute then you can print off a paper certificate to leave on the dashboard, just in case the sticker doesn't arrive in time. As Alan writes, the certificate has no expiry. We got it as we like to stay in Reims where it is required.